1. Field of the Invention
The present invention relates generally to materials and methods for the purification of gases and more specifically to an improved filter for removing and/or inactivating microorganisms in air or other gas.
2. Description of Related Art
It is common knowledge that air is generally replete with microorganisms some of which may cause human disease. The art of removing and/or destroying these infectious agents is well developed because such agents are of considerable economic importance. One obvious venue is that of medicine where the need for sterility particularly during surgical operations is well known. Less obvious, perhaps, is the need to control microorganisms and pathogens in the food industry. Not only can diseases be spread by food, accidental introduction of microorganisms leads to premature food spoilage with concomitant economic loss. Further many food products such as cheeses or wines are not sterile but, rather, are the products of microbial action. In these instances the introduction of an inappropriate microorganism will result in significant or total loss of quality. Microorganisms are also present in household dust so that the very act of "cleaning" may actually result in damage because microorganisms become inappropriately distributed.
A number of different filtration and purification technologies have been developed to deal with these and related problems. Filtration is the simplest and often the most effective. Bacteria and similar pathogens have a finite size (usually at least 1 .mu.m in their smallest dimension) and can be trapped by a filter with sufficiently small pores. The most common filter that is effective at stopping such small particles is a HEPA or High Efficiency Particle Arresting filter. These filters are often made of packed glass fibers and are effective at stopping particles greater than 1 .mu.m. However, they are less effective at stopping smaller particles and virtually ineffective at stopping particles in the size range of viruses (0.001 to 0.05 .mu.m). A further problem with HEPA filters is that they act as concentrating areas and breeding zones for microorganisms so that the filter may actually begin to add microorganisms to the filtered air.
One possible solution to this problem is to add some type of disinfectant to a HEPA filter to destroy microorganisms as they are deposited on the filter. Such a disinfectant might also inactivate viruses as they pass through the filter. The disinfectant can be either physical or chemical. For example, a very effective disinfection is achieved by heating the air to a high temperature to effectively "pasteurize" it. While such a method can be highly effective, it requires a fairly complex mechanism that consumes an inordinate amount of energy: first to heat the air; and then to cool it back to ambient temperature.
Ultraviolet light is also a very effective disinfectant and is the basis for a variety of germicidal systems. Typical of disinfectant air filtration systems is that disclosed in U.S. Pat. No. 5,523,057. In that disclosure a first filter is used to remove some particulates from air. Then an ultraviolet source is used to destroy microorganisms. Finally a carbon filter is used to remove "chemical vapors." An effective ultraviolet source generates considerable ozone which can be objectionable and must be removed by a carbon or other filters or catalysts. An electric discharge can also be used to generate disinfecting concentrations of ozone. At the same time such a discharge can be part of an eletroprecipitating filter that removes larger particulates. Again, a chemical filter may be necessary to remove excess ozone. However, the main drawbacks of an ultraviolet or electric discharge-based system is probably the size, cost and complexity of the devices.
A possible alternative is to employ a chemical disinfection system, such as the addition of a germicide to the HEPA or other filter material to prevent multiplication of trapped microorganisms. While a considerable number of such germicides are available to prevent multiplication of microorganisms, few if any are able to rapidly kill bacteria and viruses. U.S. Pat. No. 5,141,722 discloses a system that combines ozone with chlorine dioxide to effect total destruction of microorganisms. However, this chemical combination is very toxic and requires a special absorbent. Similarly, it is known that iodine is an effective disinfecting agent that might be used in air filters. However, a significant problem remains with removing the iodine vapors following the disinfection process since iodine is irritating and has a fairly distinct and somewhat unpleasant odor. Again, a typical solution is a carbon filter, but this is messy and adds considerable bulk to the filtration system.